The present invention relates generally to multilayer films, and particularly to multilayer films suitable for use in backseamed casings for packaging meat products. The present invention is particularly related to backseamed casings suitable for packaging protein-containing food products in which the film adheres to the food product, and especially to those having a relatively high protein content, also called xe2x80x98low-fatxe2x80x99 food products, such as poultry, ham, roast beef, etc. The present invention is also directed to packages.
Processed meat products, such as poultry and ham, are often packaged in a flexible, thermoplastic, heat-shrinkable film tubing commonly referred to as a casing. Although some casings have a lay-flat width of 6-20 inches, some products, such as ham, etc., are quite often packaged in a casing of smaller lay-flat width, e.g., a width of from about 3 to 6 inches. Such casings often may need to have a precisely-controlled width, because the packages are stated as having a given weight, which is uniform among packages, and the packages also have product sliced at uniform intervals, with each package containing the same number of slices. Thus, variations in casing width can result in both an undesirable degree of variation in overall package weight, as well as an undesirable degree of variation in slice weights.
Thus, there is a need for a casing having a small and uniform diameter. However, it is relatively difficult to make a narrow width, heat-shrinkable seamless casing having a precisely controlled width, using a commercially feasible process. Consequently, there is a need for some other process for making a narrow-width, precisely-width-controlled casing.
Some backseamed casings are known to be casings of small and uniform diameter. Small-diameter backseamed casings are known which have a precisely controlled casing width, i.e., a lay-flat width independent of film extrusion variations. In the production of backseamed casings (e.g., using a backseaming machine such as a Nishibe HSP-250-SA backseaming machine obtained from Nishibe Kikai Co. Ltd. of Nagoya, Japan), a flat sheet of film is folded longitudinally by passage over a xe2x80x9cforming shoexe2x80x9d. A forming shoe is a part of the backseaming machine which the film is passed under and around, i.e., so that the initially flat film is reconfigured as a tube, having a longitudinal overlap and seal therealong (lap-sealed backseamed casing), or with film longitudinal edges abutted against one another (butt-sealed backseamed casing), with the width of the tube being determined by the circumference of the forming shoe. A longitudinal lap or butt seal is then applied while the film is between the forming shoe and a sealing device, resulting in a lap-sealed backseamed casing, or a butt-sealed backseamed casing. Butt-seal casings utilize a butt-seal tape sealed to the inside or the outside surface of the casing film, along both sides of the abutting longitudinal seam of the casing film. In either event, the resulting tubing, termed a xe2x80x9cbackseamed casing,xe2x80x9d is sealed or clipped at its ends after being filled with a meat product. For some uses, the meat product is thereafter cooked while in the backseamed casing.
It would be desirable to provide a highly uniform, small diameter backseamed casing suitable for cook-in end use, the casing being made from a film which adheres to high-protein meat products, such as certain grades of ham and turkey. Of course, it would also be desirable to provide the backseamed casing with a backseam seal which survives the cook-in process.
It is known that a polar surface is needed for adhesion of a film to a meat product. Adhesion of the film to the meat is frequently needed in order to prevent xe2x80x9cpurgexe2x80x9d, i.e., cook-out, which can occur during the cooking of the meat packaged in the film if the film does adhere to the meat during cook-in. A polar film surface can be provided by using: (a) polar resin in the film layer in contact with the meat, and/or (b) surface modification, such as corona treatment, of the film surface in contact with the meat. Typically, polar polymers used for meat adhesion include: ethylene/unsaturated acid copolymer, anhydride-containing polyolefin, and polyamide.
Film-to-meat adhesion is known to be enhanced by corona treatment of the surface of the film to which the meat is to be adhered. However, corona treatment alters the film surface in a manner which can, on occasion, result in an inferior seal, i.e., a seal more likely to leak than if the film surface is not corona treated. This xe2x80x9cleaky seal problemxe2x80x9d can be avoided by xe2x80x9cbuffing offxe2x80x9d the corona treatment in the area of the seal, so that the advantageous effects of the corona treatment, i.e., greater meat adhesion, can be retained on the majority of the meat-contact surface of the film, while at the same time avoiding, in the area of the seal, the seal-quality problem caused by the corona treatment. However, the buffing step is undesirable, as it is an additional processing step which renders the casing manufacture more complex and costly. Furthermore, the buffing step is frequently inconsistent.
Since the backseaming process is generally carried out after the corona treatment, shrinkage of the film against the forming shoe (during backseaming), coupled with forwarding the film over the forming shoe after shrinkage, results in the rubbing of the film against the forming shoe edges. This rubbing reduces or destroys corona treatment, at least in the area in which the film rubs against the forming shoe. As a result, backseamed casings containing corona treated films can exhibit purge at the locations at which the film rubs against the forming shoe. Furthermore, corona treatment can be inconsistent, at least with respect to prevention of purge for products having an intermediate protein content. It would be desirable that the casing film has a consistent and adequate level of protein/meat adhesion. As a result, it would be desirable to provide a corona-treatment-free backseamed casing which prevents purge from products relatively high in protein, where the adhesion of the casing film to the meat product is uniform over the film.
Thus, it would be desirable to provide a backseamed casing of small and uniform diameter which is heat-shrinkable and suitable for cook-in end use, exhibits good purge-resistance and good seal strength, can be economically manufactured, does not produce significant meat pull-off upon being stripped from a cooked meat product, and which provides a good oxygen barrier, in order to provide good shelf life to the cooked meat product.
Heat-shrinkable films having an outer layer capable of providing meat adhesion, which are otherwise suitable for use as backseamed casings, have been found to have the undesirable characteristic of necking down on the forming shoe during the backseaming process. The necking down on the forming shoe is believed to be due to shrinkage of the film during the heat sealing step of the backseaming operation. That is, the heat sealing step can cause substantial film shrinkage in an area extending outward from the seal, causing the edges of the casing to neck down on the forming shoe. The result of necking down is a casing having xe2x80x9cruffled edgesxe2x80x9d, i.e., visible nonuniformities in the casing. In an extreme case, necking down results in the rupture of the film, as the shrinking of the film against the forming shoe places so much force on the film that the film ruptures. Thus, it would be desirable to provide a casing film which does not shrink down (i.e., xe2x80x9cneck downxe2x80x9d) on the forming shoe during the backseaming operation.
It has been discovered that the presence of an inner layer comprising a polyamide, preferably a high modulus polyamide, provides, if the polyamide layer makes up at least 5 percent of the total film thickness, a film which does not neck down on the forming shoe during the backseaming operation. Although the reasons why the inner polyamide layer prevent necking down on the forming shoe are not currently known with certainty, it is believed that various factors, including heat transfer, shrink characteristics, etc. bring about the discovered advantage of not necking down on the forming shoe. Furthermore, the inner polyamide layer also helps to provide a better quality casing film by making the casing film easier to orient, facilitating faster backseaming speeds, and also imparting enhanced seal strength, toughness, pin-hole resistance and elastic recovery to the casing film.
It has also been discovered that in the case of anhydride-containing polyolefin, if the anhydride functionality is of the order of 1 weight percent or less, the polymer often does not provide adequate meat adhesion to intermediate-protein-containing meat products, or low-protein-containing meat products. On the other hand, polymers such as polyamide can, in some instances, provide too much meat-adhesion and tend to pull meat off during unpackaging of the meat, thereby destroying the smooth surface desired upon separating the casing film from the cooked meat product, and also contributing to yield loss. Polyamides are also relatively expensive polymers. Thus, it would be desirable to provide a casing having a film providing adequate meat adhesion to prevent purge, while being able to strip the film from the meat without meat pull-off due to too much adhesion of the film to the cooked meat product. However, it has been found that adequate meat adhesion can be obtained using an anhydride-containing polyolefin having an anhydride functionality of at least 1 percent.
As a first aspect, the present invention is directed to a backseamed casing comprising a heat-shrinkable casing film. The heat shrinkable film comprises a first layer, a second layer, and a third layer, with the first and third layers being outer layers and the second layer being between the first layer and the third layer. The first outer layer serves as an inside casing layer, and comprises a first polyolefin. The first polyolefin comprises at least one member selected from the group consisting of: (i) ethylene/unsaturated acid copolymer, propylene/unsaturated acid copolymer, and butene/unsaturated acid copolymer, wherein the unsaturated acid is present in an amount of at least 4 weight percent, based on the weight of the copolymer; and (ii) anhydride-containing polyolefin comprising an anhydride-functionality, wherein the anhydride functionality is present in an amount of at least 1 weight percent, based on the weight of the anhydride-containing polyolefin. The second layer comprises at least one member selected from the group consisting of polyester, and first polyamide. The third layer serves as an outside casing layer, and comprises at least one member selected from the group consisting of second polyolefin, polystyrene, and second polyamide. The second layer has a thickness of at least about 5% of a total thickness of the heat-shrinkable casing film.
In the first layer, the first polyolefin preferably comprises an ethylene/unsaturated acid copolymer having an unsaturated acid mer present in an amount of at least 6 percent, based on the weight of the ethylene/unsaturated acid copolymer; more preferably, the unsaturated acid is present in an amount of at least 9 weight percent, based on the weight of the ethylene/unsaturated acid copolymer.
The first layer preferably further comprises a third polyolefin comprising at least one member selected from the group consisting of polyethylene homopolymer, polyethylene copolymer, polypropylene homopolymer, polypropylene copolymer, polybutene homopolymer, and polybutene copolymer. More preferably, the third polyolefin comprises at least one member selected from the group consisting of ethylene/alpha-olefin copolymer, propylene/alpha-olefin copolymer, butene/alpha-olefin copolymer, ethylene/unsaturated acid copolymer, and ethylene/unsaturated ester copolymer. Still more preferably, the third polyolefin comprises at least one member selected from the group consisting of linear low density polyethylene (LLDPE), propylene/ethylene copolymer, and propylene/butene copolymer. Yet still more preferably, the third polyolefin comprises LLDPE.
The second layer preferably comprises the first polyamide. More preferably, the first polyamide comprises at least one member selected from the group consisting of polyamide 6, polyamide 66, polyamide 9, polyamide 10, polyamide 11, polyamide 12, polyamide 69, polyamide 610, polyamide 612, polyamide 6I, polyamide 6T, and copolymers thereof. Still more preferably, the first polyamide comprises at least one member selected from the group consisting of polyamide 6, polyamide 66 and copolyamide 6/66.
The third layer preferably comprises the second polyolefin. Preferably, the second polyolefin has a vicat softening point of at least 80xc2x0 C.; more preferably, at least 90xc2x0 C.; and still more preferably, at least 100xc2x0 C. The softening point of the second polyolefin has to be high enough to undergo cook-in without causing the seals to fail (if the polyolefin is used in a seal layer). In an alternative preferred embodiment, the third layer comprises the second polyamide, with or without the second polyolefin, more preferably, as an alternative to the second polyolefin.
Preferably, the casing film further comprises a fourth layer, the fourth layer being an inner layer serving as an O2-barrier layer, the fourth layer comprising at least one member selected from the group consisting of ethylene/vinyl alcohol copolymer, polyvinylidene chloride copolymer, polyethylene carbonate copolymer and polyamide. Preferably, the second layer and the fourth layer are directly adhered.
Preferably, the casing film further comprises a fifth layer and a sixth layer, wherein: (a) the fifth layer is between the first layer and the second layer, and the sixth layer is between the second layer and the third layer; (b) the fifth layer comprises at least one member selected from the group consisting of fourth polyolefin, polystyrene and polyurethane; and (c) the sixth layer comprises at least one member selected from the group consisting of fifth polyolefin, polystyrene and polyurethane. Preferably, the fifth layer is a tie layer and comprises at least one member selected from the group consisting of modified ethylene/alpha-olefin copolymer, modified ethylene/unsaturated ester copolymer, and modified ethylene/unsaturated acid copolymer. Preferably, the sixth layer is a tie layer and comprises at least one member selected from the group consisting of modified ethylene/alpha-olefin copolymer, modified ethylene/unsaturated ester copolymer, and modified ethylene/unsaturated acid copolymer.
Preferably, the casing film further comprises: (a) a seventh layer, the seventh layer being between the first layer and the second layer, the seventh layer comprising a sixth polyolefin; and (b) an eighth layer, the eighth layer being between the second layer and the third layer, the eighth layer comprising a seventh polyolefin.
Preferably, a ratio of: (a) a sum of the thickness of the first layer and the fifth layer; to (b) a sum of the thickness of the third layer and the sixth layer is from about 0.7:1 to 1.3:1. Preferably the second layer has a thickness of from about 5 to 20 percent, based on a total thickness of the multilayer film; and preferably, the fourth layer has a thickness of less than about 15%, based on a total thickness of the multilayer film. Preferably, the heat-shrinkable casing film has biaxial orientation. Preferably, the casing film has a free shrink, at 185xc2x0 F., of at least 10% in at least one direction. Preferably, at least a portion of the casing film comprises a crosslinked polymer network.
The backseamed casing according to the present invention can be either a lap-sealed backseamed casing or a butt-sealed backseamed casing. A butt-sealed casing comprises both a casing film and a butt-seal tape film. Preferably, the butt-seal tape film comprises at least one member selected from the group consisting of polyolefin, polyamide or polystyrene, and preferably the butt-seal tape film is heat-shrinkable.
As a second aspect, the present invention is directed to a package comprising a cooked meat product within a backseamed casing. The backseamed casing is according to the first or third aspects of the present invention described herein, and the cooked meat product is adhered to a meat-contact surface of the casing film.
Preferably, the meat product comprises at least one member selected from the group consisting of poultry, ham, beef, lamb, fish, liver sausage, bologna, mortadella, braunschweiger, goat, and horse; more preferably, poultry, ham, beef, lamb, fish, liver sausage, bologna, and mortadella. Preferably, the meat-contact surface of the first layer is corona treated, and the meat product comprises at least one member selected from the group consisting of liver sausage, bologna and mortadella. Preferably, the outside surface of the casing film is also corona treated. Preferably, the meat comprises from 0-30% fat, more preferably from 1-15% fat, still more preferably from 2-10% fat, and yet still more preferably from 3-7% fat. Preferred backseamed casings for use in the package include the preferred backseamed casings in accordance with the present invention.
If a non-corona treated backseamed casing (or equivalent thereof according to the first aspect of the present aspect is used, the cooked meat product preferably comprises at least one member selected from the group consisting of turkey, ham, beef, and fish, wherein the meat product comprises fat in an amount of from about 2 to 10 weight percent, preferably 3 to 8 percent, and more preferably from about 4 to 6 percent. If a corona treated backseamed casing (or equivalent thereof) according to the first aspect of the present aspect is used, the cooked meat product preferably comprises at least one member selected from the group consisting of ham, beef, liver sausage, bologna, mortadella, horse, and goat; more preferably, the meat product comprises at least one member selected from the group consisting of ham, liver sausage, bologna, and mortadella; preferably, the cooked meat product comprises fat in an amount of from about 3 to 40 weight percent, preferably 5 to 30 percent, and more preferably from about 10 to 15 percent.
If a non-corona treated backseamed casing (or equivalent thereof) according to the third aspect of the present aspect is used, the cooked meat product preferably comprises at least one member selected from the group consisting of turkey and fish, wherein the meat product comprises fat in an amount of from about 1 to 10 weight percent, preferably 2 to 6 percent, and more preferably from about 3 to 5 percent.
As a third aspect, the present invention is directed to a backseamed casing comprising a heat-shrinkable casing film comprising: (A) a first outer layer serving as an inside casing layer, the first outer layer comprising a first polyolefin, the first outer layer having a surface energy level of less than about 34 dynes/cm; (B) a second layer comprising a first polyamide having a melting point of at least 300xc2x0 F.; (C) a third layer serving as an outside casing layer, the third outer layer comprising at least one member selected from the group consisting of a second polyolefin, polystyrene and second polyamide. The second layer is between the first layer and the third layer, and the second layer has a thickness of at least about 5% of a total thickness of the heat-shrinkable casing film.
In a lap-sealed backseamed casing according to this third aspect of the present invention, preferably the first polyolefin has a vicat softening point of at least 70xc2x0 C., more preferably at least 80xc2x0 C., in order to provide a desired level of seal strength. However, in a butt-sealed backseamed casing according to this third aspect of the present invention, the vicat softening point of the first polyolefin may be less critical. More preferably, the first polyolefin is a relatively non-polar polymer, preferably having a surface energy level of less than 32 dynes/cm.